Among the commercially available sensor technologies useful for downhole monitoring of pressure, temperature and other parameters are the Permanent Downhole Gage (PDG's) using quartz crystal technology and the Strain Gage the features of which are: require on-board electronics, are active sensors, show low reliability and higher than 3 psi/year drift in the measurement. Such equipment undergo electromagnetic interference, do not allow distributed installation (multiplexed) and are costly. For applications required to outweigh such specifications, the optical technology is the most recommended to overcome such problems.
In view of the high cost of well intervention and production interruption, the system installed in the well should have a high reliability (of at least twenty years). Optical sensor technology makes it possible to attain such a requirement.
Optical fibers bear some features such as: they can withstand high temperatures (silica up to 1,000° C.), show high transmission rates and low losses at high distances (relating to the attenuation of transmitted/received signal), as well as electrical insulation and no electromagnetic interference.
That is why it is said that optical sensors have several benefits relative to commercially available sensors, such as low hysteresis, repeatability, reproducibility, mechanical strength, twenty years working life (high reliability), passive technology, that is, no need for on-board electronics for operation, ease of implementation at large distances (longer than 20 km) without any need of a repeater for automation systems (simple telemetry at large distances), being immune to EMI/RFI noise, are of small size, operate at high temperatures and are naturally explosion-proof (low intensity laser beam approximately lower than 100 mW).
Among the available optical sensor technologies the state-of-the-art technique known as Fiber Bragg Gratings (FBG) is interesting in view of the possibility of combining a huge variety of transducers (pressure, temperature, pH, position, flow rate, etc) in a single fiber. Fiber Bragg gratings are fairly suitable to be used as sensing elements. When a Bragg grating receives a broadband (C1+C2) beam of light (laser), it reflects a narrow light band in a certain wavelength. However, the mensurands (physical parameters), such as the change in strain induced by a change in pressure or in the outer temperature, yield a change in the refractive index (temperature) and in the geometric spacing of the grating, which leads to a change in the wavelength values of the light reflected by the gratings. The magnitude of the mensurands is directly related to the wavelength reflected by the Bragg grating and can be determined by detecting the wavelength change of the reflected light.
Optical fiber sensors are optical sensors using fibers as the connecting medium for light between the mensurands (temperature/pressure) and the measurement region.
Thus, the principle for the measurement of pressure and temperature using optical fiber sensors involves injecting light using a broadband laser source through the optical fiber up to the grating and at the outlet, evaluating the main features of the returned light/wavelength and intensity that have been altered by the medium.
Among the measurement technologies, Fiber Bragg Gratings (FBG) is the one of highest potential for multifunctional and nearly distributed sensor systems for applications in the permanent downhole and reservoir monitoring.
The concept at the basis of the FBG technique is that upon designing a localized grating in a certain region of the fiber, typically of 1 to 10 mm length, according to a pre-defined wavelength, a certain wavelength of the incident light is affected (FBG filter). As a result of the medium perturbations, a change is caused in the absolute value of the wavelength, directly related to the physical parameters at the origin of such change.
Optical sensors represent a breakthrough relative to state-of-the-art systems, as have been the present quartz (PDG) sensors relative to the first strain-gauge sensors during the evolution of such systems.
The main feature of the optical sensor is the fact that it is completely passive, that is, it does not need any on-board electronics for the transduction of the measured parameter, needing only a mechanical element coupled to the FBG element, which is the fiber itself. All the electronics and optical source are placed at the surface, being easily replaced in case of failure or updated without any well work-over. The increased reliability of downhole sensors means, besides the economical advantage, the viability of other technologies, such as smart completion, reservoir monitoring (HtxHp—High temperature, high pressure wells) and artificial lift.
Optical sensors can be classified as extrinsic and intrinsic. In the first category are those where the fiber is used just to guide the light and the optical effect to be measured occurs out of the fiber. In the second case the fiber serves also as the medium where the coupling between the mensurand and the fiber occurs, this rendering this category of sensors mechanically more attractive.
Sensors can also be categorized as for the optical effect to be measured, including a change in intensity, in the polarization, in the spectrum or in the phase of the light wave.
The main features of the optical-technology sensors are:
Interferometric: those are made up of mechanical systems (microoptics), being susceptible to high intensity mechanical vibrations; do not allow multi-point installation nor high vibrations, have low reliability and high cost.
Single point FBG: do not allow multiplexing, rendering difficult downhole installation because of the number of required penetrations, are not temperature limited (above up to 150° C.).
The multi-point FBG technology, besides the above features, encompasses the possibility of placing multiple kinds of sensors connected to a single fiber (multiplexed) and placed at specific points, and further having lower cost as compared to state-of-the-art sensors.
The recent patent literature points out a few relevant documents on the multi-point FBG technology.
Thus, U.S. Pat. No. 6,016,702 teaches a pressure sensor with temperature compensation for a point where an optical fiber is attached onto a compressible bellows in a location along the fiber and to a rigid structure at a second location along said fiber, with a Bragg grating contained in the fiber between these two fiber attachment locations, the grating being under tension. As the bellows structure is compressed as a result of a change in the outer pressure, the tension on the fiber grating is reduced, this causing a change in the wavelength of the light reflected by the grating. However, temperature compensation should be carried out by isolating the temperature-monitoring grating in a pressure-isolated chamber, this adding costs to the equipment.
The sensor proposed in the '702 patent can be used alone or as a plurality of sensors, serially connected along a single optical fiber. Upon mounting the sensors in series, the optical fiber crosses a passage at the end of a bellows structure for interconnection to the following pressure sensor. The several pressure and temperature signals from the multiple pressure sensors can be differentiated using wavelength division multiplexing (WDM). Thus, each Bragg grating operates at a central wavelength λ within a wave amplitude ω that is not superimposed to the amplitude of the other Bragg grating sensors. Therefore, optical signals for the temperature and pressure of each of the sensors serially connected can be easily differentiated on the basis of the received wavelength. TDM (Time Division Multiplexing) techniques can also be used to differentiate among optical signals of different Bragg grating sensors. However, the sensor of said U.S. patent does not describe nor suggest the transducer proposed in the present application, said transducer using two Bragg gratings in a same optical fiber and having at least one of the gratings attached onto an elastic membrane.
The fiber grating pressure sensor technology taught in U.S. Pat. No. 6,278,811 comprises a pressure-detection device that can be elastically strained as a function of the applied pressure, and an optical fiber that is wrapped at least once around said device and where at least a portion of its length is fused to the device, so that the elastic deformation of the device generates a corresponding axial strain along a longitudinal axis of the fiber caused by the applied pressure. The shape of the device may be a solid cylindrical shape or either has an axial orifice formed therein. The fiber contains at least one grating impressed therein. The grating has a characteristic wavelength that changes with the applied pressure. The device is made of silica or quartz. The technology described in said patent can be used as a single sensor or as a plurality of distributed or multi-point sensors.
U.S. Pat. No. 6,519,388 teaches a Bragg grating configuration that allows the grating to be used in compression without requiring optical pins or a mechanical support structure and/or that is suitable for reducing the core to cladding coupling. According to the technology taught in this U.S. patent, a tube-encased optical fiber containing a Bragg grating comprises an optical fiber having at least one Bragg grating embedded therein, and a tube, the optical fiber and the Bragg grating being encased therein along a longitudinal axis of said tube, the tube being fused to at least a portion of the fiber in a location where at least a portion of the Bragg grating is located. The tube is made of a glass material and fused to the optical fiber on opposite axial sides of the Bragg grating. The encased grating allows the grating to be compressed without buckling the fiber.
U.S. patent application 2002/0194917, now U.S. Pat. No. 6,668,656, refers to a pressure fiber grating sensor including an optical sensing element which includes an optical fiber having a Bragg grating impressed therein which is encased within and fused to at least a portion of a glass capillary tube. A temperature Bragg grating can be used for the measurement of temperature and allow the temperature-corrected pressure measurement. The sensor can be suspended in the interior of an outer cladding by a fluid, spacers or other means.
U.S. Pat. No. 6,439,055 teaches a pressure sensor assembly for assessing the pressure of a fluid in a harsh environment including a pressure sensor suspended within a fluid filled housing. The assembly includes a pressure-transmitting device, which transmits the pressure of the fluid to sensor and keeps the fluid within the housing in a void free condition. The pressure sensor assembly maintains the sensor in a near zero base strain condition and further protects the sensor from shock and vibration. The pressure sensor assembly further includes bumpers that limit the movement of the sensor within the housing.
U.S. patent application 2003/0094281, now U.S. Pat. No. 6,913,079, teaches a monitoring system and method for monitoring a predetermined set of physical characteristics associated with a structure using the monitoring system. The system is distributed in the structure and comprises a distributed optical sensing device, further comprising a fiber optic cable; a light source operatively in communication with the fiber optic cable; a light detection device, operatively in communication with the fiber optic cable, for measuring the light received at the light detection device from the fiber optic cable; and a data processor capable of using the light measured to calculate a predetermined set of physical parameters describing the predetermined set of physical characteristics.